Silica pigments and preparation thereof



United States Patent 3,281,210 SILICA PIGMENTS AND PREPARATION THEREOFOliver W. Burke, Jr., 506 Intracoastal Drive, Fort Lauderdale, Fla., andCarey B. Jackson, Pompano Beach, Fla.; said Jackson assignor to saidBurke, Jr. No Drawing. Filed Dec. 30, 1964, Ser. No. 422,456 1 Claim.(Cl. 23-182) This application is a continuation-in-part of our copendingapplications S.N. 142, 661, now abandoned, S.N. 142,662, now Patent No.3,172,726, S.N. 142,494, now Patent No. 3,202,525, S.N. 142,495, nowPatent No. 3,178,388, S.N. 142,496, now abandoned, all filed October 3,1961.

This invention relates to the preparation of silica pigment materialsfor the reinforcement of elastomers and the like, and aims generally toimprove the same.

OBJECTS Particular objects of the present invention, severally andinterdependently, are to provide an improved process for the preparationof silica pigments for the reinforcement of elastomers and the like; toprovide an improved silica pigment having useful characteristics and atan economical cost; and to provide, inter alia, an alkaline silicapigment having improved reinforcing properties.

Other objects and advantages of the invention will be apparent from aconsideration of the herein set forth general and specific descriptionsof illustrative embodiments thereof.

PRESENT INVENTION-IN GENERAL In the preparation of silica pigment forthe reinforcing of elastomers and the like it is desirable to obtain apigment which not only imparts high tensile strength to thevulcanizates, but one which also imparts improved moduli thereto. Inpreparing such pigments by known processes these desiderata have notbeen attained, and it has been considered necessary to impart to thepigment a pH of between 5.0 and 7.2 to obtain the best approach thereto.This approach has heretofore been obtained by treating the pigment withmineral acid to a pH of about 5 and then raising the pH to the saidrange by the addition of alkali.

The present invention has disclosed that a superior reinforcing pigmentcan be prepared which is itself alkaline, e.g. has a pH in aqueousslurry of about 8.0 to 8.5, and which is nevertheless readilydispersible in elastomers, and which imparts improved tensile strengthand/or moduli to elastomers as compared to the same precipitated silicaafter-treated with mineral acid and adjusted in pH as aforesaid.

The new pigment of the present invention is produced by the extensivetreatment of precipitated silica pigment with carbon dioxide in a mannerto produce an alkaline reinforcing pigment, and the following exampleswill make apparent details of the procedures employedand characteristicsof the pigment obtained.

The silica product as wet filter or centrifuge cake may bemaster-batched with latices of natural or synthetic elastomers and/ orplastorners.

The process may be conducted in a batchwise or stepwise manner, orcontinuously, depending on selected equipment.

The novel features of the invention are set forth in the 3,281,210Patented Oct. 25, 1966 ice claim appended hereto, but the practice ofthe invention itself will be more precisely understood by reference tothe following specific examples embodying the same, which are to beconsidered as illustrative and not restrictive of the invention.

Example 1 In this example 3000 ml. of type N sodium silicate (Na O(SiOcontaining 6 moles of Na O was diluted with'6 liters of water to whichwas added 348 g. (6 moles) of sodium chloride dissolved in 8 liters ofwater and the combination was placed in a ceramic vessel and wasagitated by a 3 bladed 3" propeller driven 600 rpm. The acidulatingagent consisted of 1260 g. (15 moles) of sodium bicarbonate dissolved in15 liters of water. The acidulating agent was added to the dilute sodiumsilicate in a controlled manner according to Table I herein.

TABLE 1 Time (minutes Acidulation (moles NaHCO cumulative) cumulative) 1Initial precipitation of silica.

The precipitated silica pigment was allowed to stand overnight thenfiltered and slurried with 1 mole of sodium bicarbonate dissolved in 4liters of water and let stand 1 hour, then filtered and reslurried with4 liters of water, again filtered and then divided into 8 equal portionssome of which were treated as follows:

A portion of said filter cake was dried overnight at 0, ground, screenedand designated as silica I(a).

A portion of said filter cake was made just acid to phenolphthalein (pHrange 8.0-9.8) by addition of cc, of 2 N hydrochloric acid, after whichit was filtered and twice reslurried in 4 liters of water and filtered.The filter cake was dried, ground and designated as silica I(b).

Another portion of said filter cake was slurried in 4 liters of waterand made acid to methyl orange (pH range 3.14.4) with 2 N hydrochloricacid and brought back to an alkaline pH of 7.2 with sodium carbonatesolution, then filtered, water washed, dried, ground and designated assilica I(c).

A portion of said filter cake of water washed precipitated silicapigment was slurried with 4 liters of water and placed in an agitatedpressure vessel and while stirring left overnight with 15 p.s.i. carbondioxide pressure maintained on the vessel. The so treated product wasfiltered, water washed, dried, ground, screened and designated as silicaI(d).

A portion of said filter cake was boiled 1% hours with live steam,before treating with carbon dioxide in the same manner as with silicaI(d). The so treated silica product was designated silica 1(e).

The various silica samples 1(a) through I(e) were compounded withelastomer and compounding ingredients set forth in Table II and allvulcanizates were cured at 287 F. for 45 minutes.

TABLE II Compound ingredients: Quantities (Wt. parts) Butadiene-styrenecopolymer 1 100 Silica pigment material 58.5

as distinguished from sodium bicarbonate. The carbon dioxide used forafter-treatment converted the residual alkali in the aqueous phase tosodium bicarbonate. Dilute aqueous sodium bicarbonate solution has a pHof 8.0 to

Antioxidant2,2 methylene bis(4 methyl- 5 8.5. The same solution throughwhich carbon dioxide is 6-t.-bu-tyl-phenol) 2.0 bubbling has a pH ofbelow 6.5 due to the formation of Triethanolamine 1.0 carbonic acidtherein, and when the supply of carbo di- Paracouinarone-indene resin10.0 oxide is removed the solution pH rises with loss of carbon Stearicacid 3.0 di Xi e until the pH of 8.0 to 8.5 is again obtained. The Zincoxide 5.0 10 recovered reinforcing pigment I(d) when reslurried inDi-Z-benzothiazyl disulphide 1.25 Water (10% hOWed PH of 8.5. Thus it isevident that N,N-di-o-tolylguanidine 1.75 the prolonged after-treatmentof the silica slurry with the Sulfur 3.0 caIbOH ioxide in thepreparation of silica I(d) gradually 1 converted the bound alkali of thepigment to sodium The physical test data of the resulting vulcanizatesare 15 bicarbonate. set forth in Table III herewith. We are hereconcerned with two types of alkali of the TABLE III Bulk Density SilicaTested (in Tensile Elongation Modulus Hardness (g./ ce.) of vulcanizate)(p.s.i.) (percent) (300%) (Shore A) silica Thus the physical test dataof the silica samples 1(a) 35 pigment. The alkali carried down by thesilica precipithrough 1(2) show that the after-treatments forconverttated from sodium silicate solution, and not easily reing thealkalinity of the untreated silica prepared by the moved with water,which is conventionally called bound process of this invention produceimproved reinforcing alkali, iS SO referred lLO herein. This boundalkali 02111865 ili t i l the pigment particles to adhere together ondrying. By

It will be observed that the method of preparing silica the prolongedtreatment with carbon dioxide to form sampigment for the reinforcementof elastomers and the like ple I(d this adhesive bound alkali of theprecipitated by after-treatment of the Water washed precipitated silicasilica p1gment of Example I was converted to sodium bipigment inaccordance with Examples 1(d) and 1(2) Car nate retained by the silicapigment, and We have gave silica pigments which, When recovered andevaluated found that the so modified pigment particles are not adasreinforcing pigments in butadiene-styrene rubber vulheslve. canizate,imparted thereto an ultimate tensile strength Otherwise presented, theusual bound alkali of precipiand a 300% modulus both greater than thoseimparted by tated Silica Pigment m y be r presented by the residual thesame precipitated silica pigment after-treated by mak- 2 ill the formula2 2 ).001-.t 744 and y the presing acid to methyl orange with mineralacid and adjusting @1111 P y be regarded s having been first conto a pHof 7.2 with sodium carbonate. Verted to form The prolonged carbonationwith carbon dioxide acsioz/(Naacoalnozm cording to th1s invention can becarried out prior to the removal of a part or all of the mother liquorand it is and then to the form of course understood that suflicientcarbon dioxide must SiO2/(NaHCO3)m3 1 19 be supplied to convert all ofthe alkali metal carbonate Th th d d T in the aqueous phase to alkalimetal bicarbonate, prior to e recovere treate sl-lca Plgment may have anthe continuing treatment with carbon dioxide to convert a 3 1 contentpressed as NaHCO3 03% to about the bound alkali of the precipitatedpigment substantially 12 Preferably t0 by welght f f on or completely toalkali metal bicarbonate. It Will be apprec1ated that both the precpitated silica When the said continuing treatment is eifected in suba gthe recovered 5 1l1ca Plgment, stantial absence of the mother liquorthen it is not neces- 52 ig i to ehmmate free water, Stlu :gwtpyrewashthe resulting s1hca pigment during 1ts The term free te -i, n-mes thewater-which may Carbon dioxide may be employed as acidulating agent beremoved from the slhca Plgment by beams the P 8" in lieu of the sodiumbicarbonate employed in the forement i a tempera? of 105 to cqnstantWelght' T 0 term bound water, denotes the amount of water Which goingexample for the preparation of the s1l1ca pigment d fr tb h th t fromaqueous alkali metal silicate. Also the acidulation i 0 9 a lcafplgmen yea mg 6 plginen be ex edited b elevation of a 1gn1t1on temperature, orexample, 1000 to 1200 C., and the after treatment may p y to constantweight nnnus the amount of free water. p 'f The terms silica and silicapigment as used herein It Will be Observed that In Pf P sll}cas of referto the silica including its bound Water. The term amples R and He) thePreclpltated slllciplgmfmt a carbon dioxide includes dilute carbondioxide, e.g. prepared by acidulating sodium silicate solution withsodif h combustion f hydrocarbon materiaL um bicarbonate in an amountsomewhat in excess of the stoichiometric quantity required. to reactwith the sodium Example 2 silicate to produce silica and sodiumcarbonate, Na C0 To a stainless steel reactor agitated by a propellerstirrer was added 63 liters of water and 14,065 grams (20 moles Na O(SiOof 41 B. commercial sodium silicate. The agitator was started and thedilute sodium silicate solution thoroughly mixed and its temperatureraised to 80 C. and maintained.

Carbon dioxide was introduced through a tube-leading to the bottom ofthe reactor at a moderate rate. For the acidulation rate see Table IV.

TABLE IV [Silica preparation from N2120(Si O2)a.22 20 moles at 80 (3.]

Time, cumulative (minutes): Carbon dioxide (cumulative) (moles) 1Initial Tyndall effect.

2 Beginning of silica precipitation.

After 180 minutes the degree of acidulation of the sodium silicatesolution was 30.5% and the blue color of Tyndall elfect was observed andat this time 15 liters of water was added.

At 195 minutes or about 85 minute-s after the appearance of the Tyndalleffect the precipitation of silica commenced. At 420 minutes the silicacommenced to precipitate.

At the termination of the reaction a total of 24.2 moles of carbondioxide had been introduced.

The reactor was cooled and the silica slurry was pumped into a filterwhere it was filtered and washed until the filtrate from the aqueousslurry had a resistivity of 1700 ohms/cm.

An 1840 g. portion of the silica filter cake was dried at 105 C. toyield 292 g. of dry silica which was micropulverized and designatedcomparison sample 2(a).

Another portion of the filter cake was slurried in water. The silicaslurry was placed in an agitated glass vessel and carbon dioxide bubbledthrough while agitating for 885 minutes. Prior to carbonation a fewdrops of phenolphathalein were added to the slurry which produced anoticeable pink color. After about 5 minutes of carbonation thephenolphthalein indicator turned colorless indicating that the alkali ofthe liquor had been all converted to bicarbonate.

After 30 minutes of carbonation the first sample was taken andthereafter at intervals as shown in Table V. The solution alkalinity,precipitate alkalinity and total alkalinity were determined on eachsample. The alkalinity of the total sample was determined by boilingwith an excess of 0.1 N sulfuric acid and with methyl orange (pH range3.14.4) as indicator and back titrating with 0.1 N sodium hydroxidesolution. The alkalinity of the filtrate was determined by filtering aportion of each of the samples and titrating an aliquot portion thereofwith 0.1 N sulfuric acid using methyl orange as the indicator.

The solids for each sample were determined and the data set forth inTable V were calculated from the observed data.

TABLE V.TREATMENT SILICA SLURRY WITH CARBON DIOXIDE Time of AlkalinityCarbonation Sample (No.) Cumulative (min) Silica Silica Liquor Slurry(percent) (percent) (percent) It is to be noted that for the first 375minutes of carbonation little change took place in the amount of alkaliin the silica liquor with respect to that in the silica precipitate. By525 minutes the silica precipitate had begun to adsorb sodium carbonatefrom the solution which probably indicated that the charge on the silica(zeta potential) was changing and that the bound alkali on the silicawas substantially or totally converted to bicarbonate. After 885 minutesthe carbonation was terminated and the silica recovered by filtering andwashing the filter cake with 2 liters of water. The filter cake yieldwas 1430 g. which was dried at C. to yield 375 g. of dry silicadesignated as sample 2(b).

The silicas of Examples 2(a) and 2(1)) were each compounded withSBR-1502 in accordance with the compounding recipe given in Table VI.From the physical test results set forth in Table VII it is seen thatthe silica 2(1)) was an improvement over the silica of the comparativeExample 2(a).

TABLE VI Compound ingredients: Quantities (wt. parts) In compounding thestock the selected silica pigment material was milled into the SBR-1502together with the anti-oxidant and other compounding ingredients and thestock was aged overnight and then remilled and cured for minutes at 287F.

TABLE VII Silica Tested (in Tensile Elongation Modulus Hardnessvulcamzate) (p.s.i.) (percent) (300%) (Shore A) 2(a) Control 2, 790 720575 61 2(1)) 3, 675 700 575 60 While there have been described hereinwhat are at present considered preferred embodiments of the invention,it will be obvious to those skilled in the art that minor modificationsand changes may be made Without departing from the essence of theinvention. It is therefore understood that the exemplary embodiments areillustrative and not restrictive of the invention, the scope of which isdefined in the appended claim, and that all modifications that comewithin the meaning and range of equivalents of the claim are intended tobe included therein.

We claim:

An alkaline precipitated silica pigment having a sodium bicarbonatecontent within the range siOz/ a) nos-.119

said silica pigment having been prepared by maintaining silica pigmentcontaining bound alkali in contact with aqueous carbonic acid at a pH ofbelow 6.5 for a sufficiently prolonged period to convert the boundalkali of the pigment predominantly to sodium bicarbonate.

References Cited by the Examiner UNITED STATES PATENTS Stoewener 23182Thornhill et a1 23182 Thornhirll 23182 DAVIDSON, A. GRIEF, AssistdrntExaminers.

